1. Field of the Invention
The present invention relates to an ink-jet printing method and an ink-jet printing apparatus, and particularly to an ink-jet printing method and apparatus in which an image is formed by discharging an ink from an ink-jet head and adhering a plurality of ink dots to a printing material.
2. Description of the Related Art
A printing apparatus (ink-jet printing apparatus) which employs an ink-jet process has recently been brought into practical use as a printing mechanism in a printer, a copying machine, a facsimile apparatus or the like, or an information output apparatus in a composite electronic apparatus or a work station comprising a computer, a word processor, etc. In such an ink-jet printing apparatus, printing is performed by discharging an ink from an ink-jet head to a printing material. The ink-jet printing apparatus has the various advantages that the head can readily be made compact, a high-definition image can be printed at a high speed, the running cost is low, noise is less due to the non-impact process, and a color image can easily be printed by using multi-color inks.
Particularly, in an ink-jet head which utilizes thermal energy for discharging, an electro-thermal converter element, electrodes, liquid paths, etc. can be formed on a substrate through a semiconductor manufacturing process comprising etching, evaporation and sputtering. Thus, an ink-jet head having high-definition and high-density arrangement of liquid paths (arrangement of discharge openings) can easily be formed, and a head can further be made compact.
In a printing apparatus employing a serial scanning process in which an ink-jet head is horizontally scanned in a direction perpendicular to the direction of feeding (referred to as "vertical scanning direction" hereinafter) of a printing material, during this horizontal scanning, an image is printed by discharging an ink from a plurality of discharge openings of the ink-jet head to the printing material in accordance with image data. After printing is completed for one line by the horizontal scanning, the printing material is moved for a predetermined length in the direction of vertical scanning, and an image of a next line is then printed on the printing material in the same manner as described above. These operations are repeated to print an image over the entire printing material. The use of such an ink-jet head having a plurality of discharge openings, which can be arranged in the feeding direction of the printing material, enables the discharge openings to be arranged with a length corresponding to the printing amount for one line, i.e., the amount of feeding of the printing material. The printing speed can thus be further increased by increasing the number of the discharge openings arranged.
In the above-described ink-jet printing apparatus, when gradation printing is performed, e.g., when an image is printed, the image density can generally be determined by the density of the dots which are formed on the printing material by discharging an ink. However, when gradation printing is performed by such a process, the respective dots in a low-density portion are relatively highly visible because the dot density in the low-density portion is decreased. This consequently causes the conventional known problem that an image in the low-density portion exhibits a feeling of graininess.
On the other hand, when single-color printing is performed, it is known to be most general to decrease the feeling of graininess by using a plurality of inks having different dye concentrations and using a low-concentration ink in a low-density portion.
However, the preparation of a plurality of inks of similar colors having different concentrations for all color inks creates an increase in size of an apparatus, etc. Particularly, when an ink-jet printing process is used for a textile printing system for printing on a cloth, since this system frequently uses inks of colors other than general ink colors, i.e., special color inks, for widening the range of color reproduction, the preparation of a plurality of inks having different concentrations for these special color inks causes further increases in the number of the heads, the size of the apparatus and the cost.
Since it is necessary to use deep colors for printing on a cloth used in the textile printing system, as compared with printing on paper used in a general printing apparatus, the dye concentration of an ink used for printing on a cloth is generally increased for obtaining a sufficient density. There is thus the tendency that the number of the dots formed for reproducing a low-density portion is decreased as compared with other systems. When a low-density portion is printed by the textile printing system, therefore, a feeling of graininess is easily exhibited.